This invention relates to pressure sustaining apparatuses, and in particular to pressure sustaining apparatuses which can be used in a braking force magnification apparatus for a vehicle.
In conventional braking apparatuses for vehicles, only a foot braking force was available and so the braking effect was not satisfactory. However, in recent braking apparatuses, a greater braking effect has been required due to road conditions and other factors, and there have been an increasing number of vehicles that utilize a braking force magnification apparatus which produces a greater braking effect than can be achieved by the foot alone. A braking force magnification apparatus with pressure generating means included therein can generate a pressure different from atmospheric pressure so that the difference between the generated pressure and atmospheric pressure may be available for braking operations. Accordingly, a braking force magnification apparatus requires a pressure sustaining apparatus which can sustain the generated pressure.
One example of such a pressure sustaining apparatus used in the past is shown in FIG. 1 by a circuit diagram. In the figure, the pressure sustaining apparatus is generally shown by reference numeral 10 in which a vacuum pump 12 and a DC motor 14 connected to the pump 12 form pressure generating means. The DC motor 14 is connected to the positive terminal of a DC power source 16 through a key switch 18, and the negative terminal of the source 16 is grounded. The motor 14 is also connected to a control circuit 20 surrounded by dotted lines. The control circuit 20 is connected to ground via a sensor 22. The vacuum pump 12 produces a negative gauge pressure in a vacuum booster or a reserve tank (both not shown), the negative pressure serving as a mechanical power source for braking operations. The sensor 22 has contacts included therein as shown in the figure and is provided within the vacuum booster or the reserve tank so that the contacts are closed to provide an output signal when the absolute pressure in the vacuum booster generated by the vacuum pump 12 rises above a predetermined threshold value. The control circuit 20 includes resistors R1 and R2 connected in series between one terminal of the key switch 18 and ground. The junction of the resistors R1 and R2 is connected to one of the contacts of the sensor 22 also to the base of a transistor Q1 whose emitter is grounded and whose collector is connected through a resistor R3 to the key switch 18 and to the base of a transistor Q2 whose emitter is grounded and whose collector is connected to the DC motor 14 and to the anode of a diode D which is connected in parallel with the DC motor 14 and is connected to the key switch 18.
A conventional apparatus thus constructed closes the contacts of the sensor 22 when the absolute pressure within the vacuum booster is approximately equal to atmospheric pressure before the vehicle (not shown) in which it is installed is started. In this condition, when the key switch 18 is closed, the transistor Q1 is not conductive because its base is grounded and so the transistor Q2 is made conductive by the base current supplied through the resistor R3 from the power source 16. Therefore, the DC motor 14 is energized to reduce the absolute pressure within the vacuum booster through the vacuum pump 12. When the absolute pressure falls to a predetermined threshold value, the contacts of the sensor 22 are opened, thereby making the transistor Q1 conductive while the transistor Q2 is made non-conductive to deenergize the motor 14 or to stop the operation of the motor 14.
In such a conventional apparatus in which the vacuum booster has a small volume, as soon as the DC motor 14 is deenergized the formation of a vacuum by the vacuum pump 12 is stopped and the absolute pressure in the vacuum booster again rises above the predetermined threshold value, and the sensor 22 detects that condition. Therefore, the DC motor 14 is immediately energized again to operate the vacuum pump 12, resulting in disadvantageous hunting which adversely effects the durability of the apparatus.